If you created connection to DS1 you can not access to DS2, but you can create one more connection and concatenate the results on application level.
For example:
1. create a java class (application)
2. create connection to DS1
3. execute query (get the tables for DS1)
4. save the result on application level (in variable or array)
5. create one more connection to DS2
6. execute query (get the tables for DS2)
7. save the result on application level (in variable or array)
8. concatenate results